1. Field of the Invention
The present invention relates to a heat-sensitive recording material, and specifically, to a heat-sensitive recording material in which the softness of a base paper is made high to improve the flatness thereof while maintaining paper strength, thereby giving a high image quality.
2. Description of the Related Art
The heat-sensitive recording method, in which heat is supplied from a thermal head or the like to develop a color or record an image, is classified into the direct heat-sensitive recording method, in which heat is supplied to a recording layer provided on a support so as to record an image, and the heat-sensitive transfer recording method. The latter heat-sensitive transfer recording method is classified into the thermal wax transfer recording method, in which a transfer sheet having a heat-meltable colored ink layer is imagewise heated and then the ink is melted and adhered to an image-receiving sheet so as to form an image, and the dye-sublimation transfer recording method, in which a transfer sheet having an ink layer containing a sublimating dye is heated to diffuse and transfer the sublimating dye onto an image-receiving sheet.
In recent years, such heat-sensitive recording methods have been remarkably developed since the methods have many advantages such as that recording devices for the methods are convenient, highly reliable and maintenance free. The heat-sensitive recording material used in the heat-sensitive recording methods has, on a support, a heat-sensitive recording layer including, for example, an electron-donating dye precursor and an electron-accepting compound, or a diazonium salt and a coupler as color forming components. The color forming components advance a color developing reaction by receiving heat, and this color developing reaction is used to record an image.
As described above, heat-sensitive recording materials have a structure in which a heat-sensitive recording layer is formed on a support; therefore, the flatness of the surface of the support is a particularly important property. When the support comprises a paper substrate, in general, a softening agent or a bulk promoter is incorporated into a base paper to loosen the binding force between fibers therein, thereby making the base paper easy to deform, and then the surface of the base paper is flattened by calendar treatment. However, such a softening treatment has a problem that the mechanical strength of the base paper itself is lowered and is easily damaged.
In order to obtain a pulp sheet excellent in softness or bulkiness without damaging the strength of the base paper, the following have been suggested: a base paper comprising a bulk promoter such as an ester compound of a polyhydric alcohol and a fatty acid (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 2002-173896); a bulk promoter for paper comprising a polyhydric alcohol type nonionic surfactant (see, for example, Japanese Patent No. 3128248); a bulk promoter for paper comprising a nonionic surfactant (see, for example, JP-A No. 11-200283); a bulk promoter for paper comprising a compound represented by the formula R1COO(EO)m(PO)nR2 (see, for example, JP-A No. 11-200284); and a bulk promoter for paper comprising a specific nonionic surfactant and a specific anionic surfactant in a specific ratio (see, for example, JP-A No. 11-200285).
However, paper supports in which high levels of strength and flatness of a base paper are compatible with each other have not yet been achieved even according to the above-mentioned specific softening agent (or bulk promoter), surfactant, and composition thereof. Thus, further research and development have been desired.